Sulfur-containing monomers are good candidates for producing high refractive index optical materials, because the ready polarizability of the sulfur produces a strong interaction between the material and incident light, resulting in a high refractive index. Aliphatic thioether (meth)acrylates, monofunctional or bifunctional, such as comprise, e.g., the principal claimed matter of DE-OS 38 38 350, have an upper limit on their sulfur content (namely, according to the examples, 27 wt. % sulfur, for acetal-free structures), and accordingly the index of refraction of a polymer is &lt;1.58.
Aromatic structural elements also contribute to the raising of the refractive index, but they also contribute to increased dispersion. Low dispersion is desirable, in order to minimize the chromatic aberration of the polymer.
A higher sulfur content can be achieved, even with relatively simple structures, by means of esters of thio(meth)acrylic acid. Thus, the alkyl dithiomethacrylates described in EP-A 273,661 have a sulfur content of up to 39 wt. %. However, the ethyl sulfide partial structure (thioether-thiomethacrylate) causes problems during the production process--the yields in producing and purifying the dimercaptans used to prepare the dimethacrylates are unsatisfactory, because 1,4-dithiane forms readily, tending to form via decomposition of the higher dimercaptans. It does not appear possible to avoid purifying the mercaptan to be used as the starting product, however, because it is very difficult to purify the dithio(meth)acrylate products in an industrial process, in view of their high boiling points; i.e. at the stage at which the end product is produced, it is no longer possible to remove by-products and decomposition products.
Thus there is a continuing need for monomers with high sulfur content which are easily obtained or produced, which are colorless, and which preferably are a liquid at room temperature.
It has been discovered, in connection with the present invention, that by the inclusion of alkylidene bridges with more than two C atoms between the sulfur atoms, dimercaptans can be produced as pure products which can then be reacted to form the corresponding bisthio(meth)acrylates.